Electrical amplifier



H. BOUCKE ELECTRICAL AMPLIFIER May 13, 1941.

Filed Jan. 18, 1940 INVENTOR BY//ez'nz .Bouc-ie i@ ATTORNEY Patented May 13, 1941 2,241,581 ELECTRICAL AIVIPLIFIER Heinz Bouche, Berlin-Charlottenburg, Germany,

assigner to Radio Patents Corporation, a corporation of New York Application January 18, 1940, Serial No. 314,4;27 In Germany January 19, 1939 (C1. 17g-F71) 6 Claims.

The present invention relates to electrical ampliliers, more particularly to audio frequency amplifiers for phonograph or other sound record reproduction, and an object of the invention is to minimize or eliminate the background `noise or other similar interference experienced inamplifiers of this type.

More particularly, the invention is concerned with amplifiers of the above character embodying means for controlling the eectiveness of the interference or background noise eliminating device or system in dependence upon the amplitude or intensity of the sound signals being amplified. In this manner the noise is more effectively suppressed as the sound intensity decreases and vice versa, thereby minimizing the effect of the suppression device or system on the reproductionof the useful sound signals and preventing undesirable distortion and other impairment of the quality of the sound reproduction.

Thus, it has become known in the reproduction of phonography records to provide a needle scratch or noise filter in ycombination with an amplifier controlled in such a manner that the effectiveness of the filter is decreased in proportion .to the increase of the intensity or amplitude of the useful (sound) frequencies to be amplified. As a result thereof, the frequency transmission characteristic of such a system is practically linear for high sound intensity in which case the needle scratch noise is drowned out by the high amplitude of the sound frequencies. In case of low sound intensity, a greater or lesser cutting off of the higher sound frequencies falling within the range of noise frequencies takes place.

Practical experiments when using a filter of the aforementioned type, have shown that if a musical score consists predominantly of low notes, such as in the case of "a bass singer, the needle noise is not suihciently drowned out during the passages of high intensity but appears to be set off in marked contrast to the sound frequencies.

By the present invention the aforementioned drawback is substantially eliminated by the provision of means for controlling the effectiveness of the needle scratch or other background noise filter predominantly in inverse proportion to the intensity of the medium and high sound notes or in other Words to reduce or eliminate the effect of the low notes upon the frequency range or band width control. If the frequency range or amplifier band width is controlled by means of a lcontrolling current or potential produced from a portion. of the amplified low frequency energy by rectification and subsequent smoothing or ynltering, the above reduction of the effect ofthe lower notes of about 25-250 cycles may be accomplished by eliminating these frequencies from the low frequency energy serving for producing the control potential or by preventing admission of the lower frequencies to the rectifier circuit. This maybe accomplished by the provision of a suitable lter or a simple condenser effective in blocking the lower frequencies.

According to another embodiment, the normal control potential derived from a mixture containing the entire sound frequency spectrum is opposed by a further potential derived .from a mixture containing vthe low sound frequencies only, both control potentials being adjusted in such a manner that the portion due to the low frequency range (about 25-250 cycles in practice) of the main control potenti-a1 is compensated. Alternatively the additional control p0- -tential may be applied to a different part of the amplifier to effect a counter acting control ,for carrying out the invention.

The invention will become more yapparent by the following description taken in reference to the accompanying drawing, wherein Figure 1 showsan audio frequency amplifier circuit embodying an arrangement for automatic background noise control in accordance with the invention, andv Figure 2 illustrates a modification ofthe arrangement according to Figure 1.

Sound currents or potentials which may be derived from the output of a phonograph pick-up device, fromrthe `pick-up system of altalking picture apparatus or the like are impressed upon the primary of an audio transformer lil through input terminals a-b. The secondary of the transformer is connected in a knownmanner to the cathode I2 and grid I3 of a multi-grid amplifying valve I I further comprising a screen grid I 3' adjacent to the input or control grid I3, a positive or output grid I4, a further control or current distribution grid I5 and an anode or plate IS. The amplified sound currents in the output or plate circuit are transmitted by means of an output transformer I8 toa further ampliner or .to a suitable translating device such ,as a loud speaker to be connected to output terminals c-d.

There is inserted in theianode or plate circuit-,a lter known as a scratch or noise filter .I1 comprising, in the example shown, a series inductance and a pair of condensers shunted to ground or cathode. This filter is designed to suppress the higher sound frequencies of the needle scratch or background noise and is substantially composed to prevent their transmission to the output through transformer I3. The screen grid I 3', grid I i and the plate I6 are connected in a known manner to a high potential source indicated by the -I- signs. The output grid I4 is directly connected to the anode side of the primary of the output transformer I8. In this manner output current will flow through the transformer primary IB both from grid I 4 and plate I6, that ls, either directly or by way of :the noise or scratch filter Il, whereby the elimination of the higher notes and noise will take place in proportion to the relative distribution of the output currents upon grid I4 and plate I6, respectively. This distribution of the output current is controlled by the aid of the grid I5 placed between the grid I4 and plate I6.

In the example shown for automatic noise control, the control potential for the grid I5 is produced by applying a portion of the output energy to a rectifier I9 in series with a resistance 2t shunted by by-pass condenser 2l. Resistance 20 is connected to the cathode and control grid I5 and with the polarities of the biasing batteries in the grid circuit, the cathode lead and of the rectifier I9 being arranged as indicated it is readily seen that as the output energy or sound intensity increases, the grid I5 will become more negative, thereby blocking the electron current to the plate It and resulting in increased current flow through the grid I 4 directly to the output transformer; that is in turn decreased effectiveness of the noise or scratch filter I l.

On the other hand, as the sound intensity dee creases, grid I5 will become more positive, thereby allowing more current to fiow through the plate circuit resulting in increased effectiveness of the filter Il.

In order to render the control dependent upon the medium and high sound frequencies only as proposed by the present invention, there is provided a high-pass lter preceding the rectifier circuit I9, 2B, 2| designed to be effective in admitting only sound frequencies above about 250 cycles. In this manner, sound frequencies below 250 cycles will be prevented from exerting any effect on the control system in the manner described hereinabove.

Referring to Figure 2, there is shown a modified arrangement, wherein the amplifier which may be of a type similar to or different from Figure 1 is shown schematically by the rectangle 25. According to this embodiment, a pair of control potentials are produced by deriving output energies applied to separate rectifiers I9 and 24 having their outputs impressed upon the same circuit 20, 2l. The rectiers I9 and 24 are arranged with their polarities reversed, whereby the output potentials are opposed to each other. In order to suppress the effect of the low sound notes there is provided a low-pass filter 23 in one of the rectifier circuits, whereby only the lower notes will be suppressed by compensation andonly the medium and high sound notes are effective in producing a potential for the automatic noise control. Suitable volume control or potentiometer arrangements may be provided to adjust the relative amplitudes of the controlling potentials to effect the desired compensation, as will be obvious to those skilled in the art.

Arrangements of the above described type have the advantage of substantially improving the suppressionof the needle scratch or other background noise in sound producing systems. Moreover, the invention does not suffer from the disadvantage of increased suppression of the useful frequencies due to the continued effectiveness of the noise filter for both low and high intensity of the sound consisting predominantly of lower frequencies. This is due to the fact that in practise sounds such as music consisting predominantly of low frequencies (voice of a bass singer) are extremely poor in higher harmonics, whereby the filter is substantially more effective in suppressing needle scratch and other background noise than affecting the useful sound frequencies and impairing the fidelity of reproduction.

As will be understood, the invention has many other uses in addition to the suppression of the needle scratch noise in phonograph reproduction and in its broad aspect applies to all cases where it is desired to suppress interference or background noise comprising substantially components falling within the higher useful (sound) frequency range, such as in public address systems, talking picture apparatus (background noise due to lm graininess), magnetic recording apparatus, and many other applications. 'Ihe specification and drawing are accordingly to be regarded in an illustrative rather than in a limiting sense.

I claim:

1. In an audio frequency amplier for reproducing sound records, an electron valve comprising a cathode, an input electrode and a pair of output electrodes, an output circuit, circuit connections from each of said output electrodes to said output circuit, a filter in one of said circuit connections effective in suppressing frequencies in the higher audible range, means for controlling the relative amount of electron current to said output electrodes, further means for producing a potential varying in proportion to the average amplitude of the sound currents being amplified, said potential controlling said last mentioned means, whereby with increased sound intensity the relative current to the output electrode connected to said output circuit through said filter is decreased and vice versa, and further means whereby said control potential varies only in proportion to the average intensity of sound signal components beyond a predetermined lower limit frequency.

2. In an audio frequency amplifier for reproducing sound records, an electron Valve comprising a cathode, an input electrode and a pair of output electrodes, an output circuit, circuit connections from each of said output electrodes to said output circuit, a filter in one of said circuit connections effective in suppressing frequencies in the higher audible range, means for controlling the relative amount of electron current to said output electrodes, further means for producing a potential Varying in proportion to the average amplitude of the sound currents being amplified, said potential controlling said last mentioned means, whereby with increased sound intensity the relative current to the output electrode connected to said output circuit through said filter is decreased and vice versa, and further means whereby said control potential Varies only in proportion to the average intensity of sound signal components higher than 250 cycles per second.

3. In an audio frequency amplifier for reproducing sound records, an electron valve comprising a cathode, an input grid, a positive grid and an anode, an output circuit connected to both said positive grid and anode, a lter inserted between said anode and output circuit, said filter being effective in suppressing frequencies in the upper audible range, a further control grid positioned between said positive grid and anode adapted to control the relative electron current iiow to said positive grid and to said anode in dependence upon a biasing potential applied thereto, means for averaging a portion of the output energy for deriving a potential proportional to the averaged energy to be impressed upon said further grid, whereby with increased output the relative current to said anode is decreased and vice versa, and further means whereby said control potential varies only in proportion to the average strength of output current components above a predetermined lower limit frequency.

4. In an audio frequency amplier for reproduclng sound records, an electron valve comprising a cathode, an input grid, a positive grid and an anode, an output circuit connected to both said positive grid and anode, a filter inserted between said anode and output circuit, said filter being effective in suppressing frequencies in the upper audible range, a further control grid positioned between said positive grid and anode adapted to control the relative electron current flow to said positive grid and to said anode in dependence upon a biasing potential applied thereto, meansv for averaging a portion of the output energy for deriving a potential proportional to the average energy to be impressed upon said further grid, whereby with increased output the relative current to said anode is decreased and vice versa, and further means Wher-eby said control potential varies only in proportion to the average strength of output current components above a lower limit frequency of about 250 cycles per second.

5. In an amplifier for signal energy comprising a band of component frequencies, an electro-ri valve comprising a cathode, an input control electrode and a pair of anodes, means for impressing signal potential upon said input electrode, an output circuit, coupling circuit connections from each of said anodes to said output circuit, filter means in one of said circuit connections effective in suppressing frequencies in the higher frequency range of said band, a further control electrode for varying the relative amount of electron current to said anodes, a rectifying circuit, means including a filter effective in suppressing frequencies in the lower frequency range of said band for impressing signal energy upon said rectifying circuit, further means including a smoothing lter to derive a control potential from said rectifying circuit varying in proportion to the average amplitude of the signal energy being amplified, and means for impressing said control potential upon said further control electrode.

6. In an amplifier for signal energy comprising a band of component frequencies, an electron valve comprising a cathode, an input control electrode and a pair of anodes, means for impressing signal potential upon said input electrodes, an output circuit, coupling circuit connections from each of said anodes to said output circuit, lter means in one of said circuit connections effective in suppressing frequencies in the higher frequency range in said band, a further control electrode for varying the relative amount of electron current to said anodes, a first rectifying circuit, means including a filter effective in suppressing frequencies in the higher frequency range of said band for impressing signal energy upon said rectifying circuit, a further rectifying circuit, means for impressing signal energy of substantially the entire frequency range of said band upon said further rectifying circuit, further means including smoothing lter means to derive and differentially combine potentials from each of said rectifying circuits varying in proportion to the average amplitude of the respective impressed signal energies, and means for applying the derived differential potential to said further control electrode.

HEINZ BOUCKE. 

